The interaction of native and introduced fruit consumers (especially the squirrel Atlantoxerus getulus) with native and non-native fleshy-fruited plant species was studied in the semi-desertic Fuerteventura Island (Canary Islands). The ecological effect of the A. getulus squirrel was compared to that of another introduced mammal (the rabbit Oryctolagus cuniculus) and a native seed disperser (the lizard Gallotia atlantica). Fleshy fruits were an essential food and water resource in this xeric island. Coinciding with maximum fruit availability, consumption of native plant fruits occurred mainly in the spring while introduced plants were ingested in autumn. A significant number of Rubia fruticosa fruits were consumed by lizards, whereas squirrels ate a large amount of Lycium intricatum fruits. Asparagus pastorianus was consumed in similar quantities by each of the three fruit consumers. Fruits from Opuntia were mainly eaten by the squirrels. Lizards should be considered as legitimate seed dispersers for the three native species, while the two mammals are illegitimate dispersal agents. However, in the case of the non-native Opuntia, squirrels produce an invasional meltdown effect in the colonization of this cactus on Fuerteventura Island. While this invasive squirrel plays a significant negative predatory role on native seed plants, it is an effective disperser of some introduced plants. Thus, it constitutes an appropriate example from which to elucidate the mechanisms underlying the disruption impacts of introduced species in island ecosystems.

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Marta López-Darias has a FPU grant financed by the Spanish Ministry of Education and Science. Fernando Hiraldo has always supported our work. The Cabildo Insular de Fuerteventura allowed us to work in the protected badland area and gave all kinds of logistic support. This study was partially financed by La Obra Social de La Caja de Canarias and the Project CGL2004-04884-C02-01/BOS, funded by the Spanish Ministry of Education and Science and partially supported by Feder funds from the European Union.